(a) Field of the Invention
The present invention relates to a defective circuit correction method and apparatus, and more specifically to a laser correction method and apparatus for correcting fine patterns on substrates such as a photomask for forming an LSI.
(b) Description of the Related Art
A conventional laser correction apparatus has a laser-irradiating microscopic system disposed above the substrate for irradiating a laser. The substrate is irradiated by a laser from above to evaporate a Cr film on defective spots for removal. A conventional laser correction apparatus is described in Japanese Patent Laid-Open Publication No. Sho. 56-111227, entitled xe2x80x9cLaser Working Device with Projection Method Using Iris Diaphragmxe2x80x9d. This method uses a high precision defect machining technique with a laser irradiation unit added to an ordinary microscopic system. A laser beam, which is collimated with slits, is transmitted through an objective lens from above the substrate, and then the image of the slits is focused on the pattern formed on the substrate.
On the other hand, Japanese Patent Laid-Open Publication No. Hei. 7-104459 entitled xe2x80x9cDefect Correction Method and Device for Photomaskxe2x80x9d, describes a method of removing by evaporation a Cr film on the substrate by directing the pattern surface downward and then irradiating the pattern with a laser penetrating the substrate from the rear side of the substrate. The described technique indicates that the re-deposition of evaporated fine particles onto the substrate is suppressed since the pattern surface is directed downward and the evaporated particles fall downward due to gravity. Besides, it is recited that there is another advantage of suction of falling fine particles with an exhaust mechanism installed along the optical axis under the substrate.
The former conventional laser correction technique by which the substrate is irradiated by a laser from above has a problem in that evaporated fine particles may re-deposit onto the substrate and cause defects. This problem has not been identified because such defects were beyond measurement in the current technique. However, since the semiconductor structures have become smaller than ever, even fine particles as small as around 0.1 xcexcm that were not recognized heretofore as causing specific defects are recognized now as a source of defects. Thus, there appears to be a need to correct such defects. Consequently, it becomes necessary to reduce the re-deposition of fine particles during the laser correction process.
On the other hand, the latter conventional method for correcting detects by irradiating the substrate with a laser from above, with the pattern surface being directed downward, has another problem. The problem is that the precision of a laser image formed deteriorates because the substrate is irradiated by the observation light or laser light, which causes parallel plate aberration of the substrate. Also in this case, it is difficult to observe fine patterns from the rear side of the substrate. The deposition of relatively large fine particles of 0.2 xcexcm or larger onto the substrate is almost completely prevented by installing a suction port under the substrate. However, there is still a problem in that some of the fine particles smaller than 0.2 xcexcm are observed to re-deposit onto the substrate within a 20 xcexcm distance from the laser-irradiated spot since these particles take time to ride on the suction stream and consequently stay near the substrate.
It is, therefore, an object of the present invention to provide a laser correction method and apparatus that can prevent re-deposition of fine particles created by the laser irradiation of the substrate, substantially without degrading the laser focusing characteristics and without reducing the ability to observe the fine patterns.
Another object of the present invention is to provide a laser correction method and apparatus that can prevent the deposition of fine particles created during the laser correction onto optical components such as an objective lens. A further object of the present invention is to provide a laser correction method and apparatus can handle both opaque and clear defects.
The present invention provides a laser correction apparatus including: a laser source for emitting laser; an optical system having an objective lens for irradiating upward the laser and focusing the laser onto a defect on a thin film formed on a bottom surface of a substrate; a window port interposed between the bottom surface of the substrate and the objective lens, the window port including a top surface opposing the bottom surface of the substrate with a space therebetween and having a central opening for allowing the laser to pass therethrough, and an inner wall including an upper portion extending from the central opening and a lower portion having a larger diameter than the upper portion, the lower portion having a plurality of gas outlet ports therein; a gas source for introducing a gas to the space between the bottom surface of the substrate and the top surface of the window port from a periphery of the top surface toward the central opening, the central opening allowing the gas to pass therethrough toward the inner wall; and a gas exhaust section for exhausting the gas through the gas outlet ports.
The present invention also provides a method for correcting a defect on a thin film formed on a bottom surface of a substrate by using the laser correction apparatus of the present invention, the method including the step of irradiating a focused laser onto the defect through the central opening while blowing a gas to the space from a periphery of the top surface and exhausting the gas through the central opening.
In accordance with the laser correction apparatus and method of the present invention, the particles generated by the laser irradiation can be removed by the gas flow penetrating the central opening and then advancing downward in a spiral flow toward the gas outlet ports.